1. Field of the Invention
The present invention relates to the technology of magnetic resonance (MR) and, particularly, to a method and an apparatus for correcting the uniformity of a magnetic field in a magnetic resonance system.
2. Description of the Prior Art
In current magnetic resonance (MR) systems, in order to obtain a magnetic resonance image with relatively good quality, a relatively high uniformity (homogeneity) of the basic magnetic field is required, the basic magnetic field being generated by a basic field magnet. Generally, there are no problems with the uniformity of the magnetic field in the central area of such a magnet (i.e. an annular area), while the uniformity of the magnetic field will decline significantly in the non-central areas. For acquiring MR data from certain tissues and organs, such as the shoulder or chest, etc., since a patient lies flat on the patient bed, which passes through the magnet, these organs are located in the non-central areas of the magnet and, accordingly, the quality of the scanned magnetic resonance images of these tissues and organs is relatively poor, thus influencing the doctor's subsequent analysis and diagnosis. Therefore, in practical applications, it is necessary to adopt certain measures to correct the uniformity of a magnetic field in the non-central areas of a magnet.
If the uniformity of a magnetic field needs to be corrected for all the non-central areas of a magnet, this is generally quite difficult to accomplish and is associated with relatively high costs. Therefore, in practice the uniformity of a magnetic field is generally corrected only for a specified area, for example, if the shoulder-of a patient needs to be scanned, then the uniformity of the magnetic field is corrected only for the area related to the shoulder scanning.
Passive shim shells (PSS) are usually used for correcting the uniformity of a magnetic field in the prior art. The outer casing of such a passive shim shell is made from materials such as plastics, etc., with a number of shims being provided therein. The materials of the shims are generally iron or other ferromagnetic materials. After a passive shim shell has been placed into a magnetic field, the uniformity of the magnetic field is changed by utilizing the characteristics of the shims. A particular implementing method is as follows. Since the spatial distribution of the magnetic field in different areas is known, the placement position of the passive shim shell in the specified area and the number of the shims needed to be placed can be calculated on the basis of this information. An operator then can place the number of shims in the passive shim shell according to the calculated results and place the passive shim shell at a proper position. The passive shim shell can be fixed by a bracket connected to the patient bed.
However, there are certain problems in the aforementioned method in practice because the specific position of the bracket is placed by the operator according to the calculated placement position for the passive shim shell. While there are inevitably certain errors as a result of human operations, it is impossible to ensure that the position of the bracket fully agrees with the calculated placement position of the passive shim shell, thus resulting in the case that the uniformity of the magnetic field is not properly corrected. Furthermore, the size of each shim also influences its performance and if the actual size thereof is slightly different from the theoretical size when the shims are manufactured, then the result of the correction by the passive shim shell to the uniformity of a magnetic field will be subsequently affected. In summary, the currently available method cannot properly achieve the correction of the uniformity of a magnetic field, thus affecting the imaging quality of a magnetic resonance image.